Electronic transfer unit



Feb. 20, 1951 R. 0. Fox ET AL 2,542,807

ELECTRONIC TRANSFER UNIT Filed Oct; 24, 1946 05c 17/0 f r L1 3 i /z WITNESSES! INVENTQRS I Boy Uz 6x and g FFea er/bafiea/e, 0-

W BY

ATTORNE Patented Feb. 20, 1951 ELECTRONIC TRANSFER UNIT Roy 0. Fox and Frederic S. Beale, Baltimore, Md, assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application October 24, 1946,,Serial No. 705,362

1 8 Claims.

.Our invention relates to electronic switching devices and, in particular, relates to switching devices of the type sometimes known as transfer units which are used on two-way teelphone systems to cause alteration ,of'connections of a carrier transmitter and recevier at each station to occur automatically in response to incidence of a users speech in the microphone. In other words such transfer units automatically change the circuit connections as two users at different stations alternately speak :into their respective microphones in carrying one two-way conversation. Onewidely' used system of this type is the carrier telephone systems employed by opera-- tors of electric-powersystems inwhich the power transmission line-itself furnishes the channel for thecarriercurrents. .A system of this type, upon which the present invention can be employed withimproving efiect, is shown in United States Patent No. 2,310,060, Boothand Bock, issued February.2, 19.43, for Two-Wire Radio Transmission, and assigned tothe assisneeoi the present appli- 811310 1 The aforesaid Patent No. 2,310,060 describes a two-way oarrier=current telephone system in which each station comprises a transmitter includingia generatorof carriercurrent modulated by a local microphone and a receiver to demodulate a carrier current message coming in over the line. Since the transmitter and 'receiverat each station are connected to the same transmission line, it is necessary that while speech into thelocal microphone at any station is mod ulating the carrier-current generator there, the lo al vreceiver shall be cut ofi from the line to prevent its repeating and reverberating the message which is being sentout at the moment by that station. Itis further necessary that when carrier current isbringing in a messagefrom a distant station, the receiver be fully energized to demodulate it,,b,ut that the local transmittershall be cutofi from the line so that itcannot impose on the latter a confusing signal. In short, switching functions must be performed on both the transmitter and the receiver at each station alternaely to fit the system for two-way conversation.

In the aforesaid patent, this switching is achieved by providing, when no signal is being either received or transmitted, the local oscillation generator (numbered 102 in the patent), constituting a part of the transmitter, and an audio amplifier (55), constituting part of the receiver, with negative bias voltageswhich make them non-conductive; while at the same time a modulating'amplifier (101 and 153), acting on said oscillator,.and a -radio frequency amplifier (l3) constituting part of the receiver, are so biased as to be conductive and operative. Then, when-a user speaks into the microphone at any ill? station, the aforesaid negative bias is removed from the local oscillator 102, thus sending out carrier current over the line to a called station. The resulting reception of this carrier current at the distant station causes the removal of the negative bias voltage blocking the audio ompliher at that station, thus permitting the user there to hear the message impressed on the line by the sending station. At the same time, a new negative bias voltage is developed by the microphone output at the sending station and impressed on the local receiver radio frequency amplifier (13) to block it and render it temporarily unresponsive to the current being impressed on the line by the local oscillator there. Likewise, a second new negative .bias voltage is developed at the distantstation by the current entering the receiver there from the line, and this.

bias voltage isimpressed on the modulating amplifier (101) at that station to render it nonconductive and inoperative .to impress any words which might be spokeninto themicrophone there on the transmission line as long as a message is coming in. In other words, once a given station attains control of the transmission. channel by impressing a message thereon, it cannot be interrupted by a speaker at the receiving station until the first station ceases to speak Cessation of speech at the first station causes disappearance of the microphone-current to reimpose the negative blocking bias on the local oscillator there, thus cuttingoii carrier current from the transmission line. Consequently, the entire transmitting system returns to its initially-described condition which will hereafter be referred to by the term normal. Resumption of speech at either station then carries out again the above-described sequence of switching operations.

It will be seen from the above that to conduct these switching operations, each station requires two sets of negative bias voltages, each set comprising a first voltage which is substantial while a control current is zero and which is reduced to zero when-the control current comes into existence, and also a second voltage which is zero while the control current is zero and which become substantial when the control current comes into existence. For one of these sets of bias voltage, the control currentis the output of the local microphone; for the other set, the control current is carrier current coming in over the transmission line from the distant station.

One object of our invention is, accordingly, to-provide a circuitvfor supplying such sets of two negative bias voltages as are described in the preceding paragraph.

Another object of our invention is to provide the iabove-described sets of bias voltage in such a way that the initiation of new of the control current causes the normally existing negative biasing voltage to fall to zero only slowly, but the normally zero bias voltage to rise very rapidly.

Another object of our invention is to provide the above-described sets of bias voltage in such a way that the one which is normally substantial and falls to zero when the control circuit is energized does so after the rise of the nor- .mally zero bias voltage.

Another object of our invention is to provide sets of the above-described bias voltages in such a way that the one which is normally substantial and falls to zero upon the energization of the control circuit does so as a result of the rise of the normally zero bias voltage. 7

Still another object of our invention is to provide the above-described sets of bias voltage which are of such a type that the rise from zero after the disappearance of the control current of that bias voltage which is normally substantial shall occur before the fall to zero of the other bias voltage which is normally zero" 7 A still further object of our invention is to provide bias voltages which come into existence so quickly after energization of the control circuit that the complete switching of the system occurs in a time of the order of 50 milliseconds; that to say, the telephone system may be switched on and off between words of ordinary speech. Such switching is faster than the brain responses of the user.

A still further object of our invention is to provide a system in which, while the transmitter is switched onto the line with high speed after the initiation of speech, the switching ofi of the transmitter after cessation of speech is subjected to a very substantial time delay, and the ratio of the time delay to the initiating period can be varied at will by operating certain manual switches provided in thesystem.

Other objects of our invention will become api parent upon reading the following description, taken in connection with the drawing in which the single figure is a schematic diagram of a preferred circuit for producing the bias voltages described above in connection with our invention.

Referring in detail to the drawing, the switching operations carried out by the transfer unit at each station are powered by a local oscillator I having a frequency above the audio band-pass limits of the associated equipment. Alternating voltage from this oscillator is impressed by a secondary winding 2 upon the anode of an electrical discharge tube 3 which is preferably, although not necessarily, of the gas-filled, or Thyratron, type. The. cathode of the tube 3 is connected through a: resistor 4 shunted by a capacitori to the negative line which is con nected to the negative terminal of a direct-cur-.

rent voltage source (not shown). The positive terminal ofthis voltage source is likewise connected to the cathode of the tube 3 through a resistor 6 providing the bias necessary to render A second secondary winding II, energized by the oscillator I, is connected through acurrent limiting resistor I2 to the anode of another elec-- trical discharge tube I3 which is preferably similar to the tube 3. The cathode of the tube I3 is connected through a resistor I4 to the opposite end of the secondary winding II. The terminal of the secondary winding 2 is connected to the cathode of a rectifier which may, for example, be a diode I5, and is also connected through a suitable limiting resistor I6 to the positive terminal of the above-mentioned direct-current voltage source. The cathode of the diode I5 is connected through a suitable resistor IT to the control electrode of the tube I3. It is also connected through a capacitor I8 to the negative; line. The anode of the diode I5 is connected, to said negative line through a resistor I9, and the common terminal of the secondary winding II and of resistor I4 is connected to said negative line through a capacitor 28. The cathode of the tube I3 is directly connected to said negative line 20A, which is connected to' act as the ground or negative bias of the controlled transmitter and receiver system.

It will be noted that the secondary winding I I is connected through resistors M'and I2 between the anode and cathode of the tube l3 and that the direct-current voltage source above-men- V tioned impresses its voltage through the resistors I6 and I1 to make the control'electrode of tube I3 positive relative to its cathode. Under such circumstances, when no control voltage is impressed on the control electrode I of tube 3, tube I3 is conductive, rectifying the output of secondary winding, to produce a direct-current voltage drop across the resistor I4, making the a lower terminal of the latter negative relative to the cathode of tube I3. The resistor I4 may, accordingly, be used as the source of negative bias voltage which is normally in existence when no control current is being impressed on the input circuit of tube 3. In other words, the lower terminal of the resistor I imay bethe source of negative bias potential which is required when the telephone system is in its normal, or unenergized, condition.

In contrast to tube I3, it will be noted that, while the control electrode 1 of tube 3 is at the potential of the negative line when the control voltage is zero, the cathode of tube 3 is made positive relative to said negative line by the current flowing serially through the resistors 4 and Ii from the positive terminal of the direct-current source. The ratio of the resistors I and 6 is made such that the voltage impressed on the anode of tube 3 by secondary winding 2 through resistors 8 and I6 and the positive terminal of the aforesaid direct-current source is insufiicient to cause current flow through tube 3 when the control electrode I thereof is at the potential of the negative line. Since the anode of the diode I5 is at the potenial of the negative line, while the cathode thereof is connected to the positive line through resistor I6, no current flows through the diode I5 when tube 3 is non-conductive.

If now an incoming signal impresses a voltage on the control electrode 1 to make it positive, the circuit constants are so'proportioned that tube 3 immediately becomes conductive. Resistors E q and I6 are large compared to the resistance of elements -5, I5 and I9. The secondary winding 2 then sends current through resistors 8, 3, I9 and tube l5, thereby producing such a voltage drop through the resistor 'l9 "that the .control electrode of tube -|3"i's made sufiiciently negative to cutoff current fiowthrough resistorrl4. Since tube I is conductive the cathode of the tube. is in effect connected to the (how) negative-termin-a1 of resistor lfl'and "thegr-id .of tube I3 is negative relativeto its cathode. Tube 13' therefore becomes non-conductive. However this event occurs only after a control potential'appears 'atthe upper terminal of resistor 19. The upper terminal of the resistor [9 may thus be used as a source of bias potential which is zero (because there is normally no current flow through resistor I9 while no signal is impressed: on the input-of tube 3,'but which becomes negative when a signal appears at the input of tube 3.

To summarize then, the lower terminal of resistor I4 and the upper terminalof resistor l3, respectively, provide the pair of negative bias voltages required to effectswitching of the re-- ceiver and transmitter as described in the introductory paragraphs; namely, the lower terminal of resistor i l provides a bias voltage which is normallynega'tive relative to the negative line of the system, butwhich become's zero when a control current appears at the input of tube 3,. while the upper terminal of resistor 1 9 provides a bias voltage which is normally zer'orelative to the negative line, but which acquires a substantially negative value relative to that line when the control currentappears in the input of tube 3. A lead 2| from the lower terminal of resistor 14 may normally impress a blocking 'voltage on the audio amplifiert55 in Eatent"2,3l0,- 060) which disappears when incoming signals impress control voltage on the input of tube3, and a lead 22 from the upper terminal of resistor l9 may be used to impress azero bias voltage on modulating amplifier (101, 153 in Patent 2,310,060) allowing it normally 'to be conductive, and to impress ablo'cking bias voltage on it when a line signal appears at the-input of tube 3.

The capacitor l3 and resistor II; are soproportioned thatcurrent flow through tube 3' and resistor l6 starts very quickly after control voltage is impressed on control-electrode 1, and the cathode of diode I 5 becomes negati-ve'relative to its anode, thereby producing the negative bias voltageracross resistor I 9 at *thesame instant that the control-electrode -of tube I 3 becomes negati-ve to its cathode. Therefore, interruption of current flow in tube l3 follows after, andresults from, the: production of bias voltage at resistor I9. The disappearance ofbia's at terminal '2! occurs still'later because capacitor has to discharge through resistor l4 before such disappearance takes place. Thus the'au-dio amplifier (55) is unblocked only after, andas a result of, the impressionof blocking bias on the modulating amplifier (.101, 153) Similarly upon cessation of the control voltage at control-electrode -l current'flowthrough tube 3 ceases at once, and the low time constant of capacitor l 8-with resistor l6 permits the potential at the control electrode of tube -l3 and at the cathode of diode I5 lto become positive relatively quickly. Current flow thus-starts quickly in tube 13 to impose'negative bias voltage: on'ter-minal 2| but, the nonconductive condition of diode 13 isolates the re sistor I9 and" the control-electrode of the modula'tor amplifier (101, 153 in Patent 2,310,060) from the 'restof thesystem. A capacitor '23- shunting said modulator control-electrodeis-so proportioned to resistor M as to ha-ve a long 6* moved 'Irom'the modulator-onthe local. transmitter onlyafter the blocking voltage .on the local audioxamplifier .(55 inPatent 2,310,060) has been re -applied.

The foregoing .dESCI'ibBSlJhE-i transfer unit suitablefor employmentt in the receiver of each station. .Howevemthe switching unit for the'transmitter portion of each station. is made somewhat more elaborate =by interposing between the control current, which of course is the output current of the :local microphone, and the input of. tube 3a timing circuit of the type described in the applicati-on .of vR. C. Fox, entitled Timing Mixerflfiled concurrently herewith on October 2.4, 19 16,?bearing .Serial No. 705,363, and now Patent Number-2,479,335 assigned to the assignee of the present application. The timing unit above mentioned comprises the portions of Fig. l to theleft of the discharge tube 3. In short,

.instead of impressing on the input of the disof "the drawing,-and discharge tubes 3| and 32,

together with the associated circuit network, modify the control voltage-in such a way as to improvethe performance of the telephone system in responding to speech input to the local microphone.

Referring. in. detail to the drawing, the output of. the microphone at the local station, suitably amplified if desired, may :be impressed on. the primary winding .3.3-having,1a pair of secondary windings 34 and 35, respectively shunted by capacitors36 and 31. Capacitors 36 and 31 together withwindings 34 and 35 constitute resonant circuits at a frequency of approximately 1500 cycles per second. The resonant circuits peak the response of the audio amplifier system and aid. in providing a satisfactory band pass characteristic.v Terminals of the windings 34 and 35, which. have opposite polarities at any instant are respectively connected to the anodes of the tube 3|, while the other terminals of the windings 34 and '35 are connected together to the negative. line. terminal of the system. The cathode 38 of the tube 3| is connected through a capacitor 39 to the aforesaidnegative line terminal, and the cathode 4| .of the tube 3| is connected through a capacitor 42 to the same negative line. The cathode 4| is connected to thesaid negative line through a resistor 43 and is likewise connected 'to the cathode of a rectifier which maybe a diode 32'. The cathode of the diode 32 is likewise connected to said negative line through a resistor 44 .in series with a capacitor 45 which is shunted by a resistor 46. The anode of'the diode 32is connected to the cathode 38. The cathode 38 is likewise connected to one pair of terminals of each of a pair of twoway switches 41, 48. The hinged terminal of the switch 41 is connected to the negative line through 'acapacitor 49 and is connected to the hinged terminal of the switch 48 through a sec ond capacitor 5|. A second contact terminal of the switch 48 'isconnected to the negative line, while the second terminal of the switch 4? re mains unconnected.

The resistor 46 constitutes the output terminal ofthe timing device above described and impresses"=a-voltage between the negative iineand thecontrol electrode 1 "of the tube 3.

Where, as in the present case, it is desired that the transmitteroutput beswitched onto the transmission line ver quickly afterbeglnning of speech in the local microphone of a station, but that the transmitter be switched off only after a substan tial time delay following the cessation of speech. in the said microphone, the capacitor 42v is made more rapidly than the latter, thereby making the cathode of the tubfe 32 quickly become more positive than its anode. 'No current can, therefore, fiow through the tube 32 and the capacitor 02 will charge up quickly to the voltage of the winding 35 to be followed'only slowly by the capacitor 39. With the switches 41 and 43 in the position shown in the drawings, the capacitors 49 and 51 are connected in series with each other to form a shunt circuit adding to the capacitance of the capacitor 39. While the capacitor 4-2, being in shunt with' the resistor 43, discharges slightly during negative half periods of the winding 35, the value of the resistor 43 is made such that such discharge is only moderate.

The voltage across resistor 43 is impressed through resistors 44 and 46 and capacitor 45 on the input of tube 3. Hence the timeneeded to make the control electrode i positive and initiate conductivity in tube 3 after inception of speech currents in primary winding 33 is determined by the speed with which capacitor 42 charges up.

This speed may, by making capacitor 42 sufficiently small, be made very high. Capacitor 42 will remain charged to nearly the voltage impressed by winding 35 until cessation of speech currents in the winding 33. In the meantime, capacitor 39, together with capacitors 49 and have been charging more slowly through tube 3 l but their value may readily be made such that they will acquire substantially the same voltage as capacitor 42 before even a short sentence has been spoken into the microphone. Cessation of microphone current ,will stop the charging of v resistor 43; hence the voltage impressed on the input of tube 3 after cessation of speech into the local microphone may be given as great a time delay as desired by making the size of capacitors 39, 4.9 and 5| great enough. In this way, switching voltage may be applied as rapidly as desired to the local transmitter upon inception of speech to the local microphone; but any desired time delay may be permitted between words or even. sentences before the local microphone will lose exclusive control of the transmission line.

It will be evident that by moving the switch 41 to its other contact position, the capacitor 5i may be short-circuited and the capacitor 49 be connected directly in multiple to add a still greater amount to the capacitance of capacitor 33. On the other hand, by leaving switch in its upper position and moving the switch 48 to its lower position, the capacitors 49 and 5| are,

in effect, removed from the circuit, while with the switch 4'! moved to its'lower position capacitor 45 is connected in parallel with capacitor 33. Since the value of capacitance in these networks determines the ratio of the time delay, to the initiating time for the control voltage impressed on tube 3, the capacitor network 49,- 5I and switches 41 and 48 provide means of substan-- tially altering at willthe' value of this ratio.

There will, of course, be provided for each station, two different control networks, such as we havesabove described; namely, the control network comprising the portionto the right of the input circuit of tube 3 will be provided for thei receiver section of the station, and the complete network shown in Fig. 1 willbe provided for the transmitter portion of the station. There will, therefore, be included two sets of secondary windings, such as 2 and II; but these may ob viously be fed by a single oscillator i.

In the transmitter section of the station, the

resistor corresponding to resistor M in the drawing has its voltage drop used to normally impress a blocking voltage on the local transmitter osthe case where tubes 3 and I3 control the switching of the receiver, the resistor l4 connected to control the local oscillator may be made to normally block said local oscillator (tube 101 in Patent 2,310,060) until after resistor H) has blocked the receiver amplifier (tube 13 in Patent 2,310,060) and to reblock said local oscillator before resistor l9 unblocks the receiver amplifier.

We claim as our invention:

1. In combination with an electrical discharge tube having a pair of principal electrodes and a control electrode, a first source of voltage con nected to send current through said tube, a bias resistor in series with a direct-current source,

means to bias said control electrode in response to'the combined voltage drop across said directcurrent source and bias resistor, a second source and means for controlling flow therefrom of current through said bias resistor, and a rectifier and a serially connected limiting resistor in shunt with said direct-current source and said bias resistor, said rectifier being poled to prevent current flow from said direct-current sourc through said limiting resistor.

2., In combination with an electrical discharge tube of the gas-filled type having a pair of principal electrodes and a control electrode, a first source ,of voltage connected tosend current through said tube, a bias'resistor in series with a direct-current source, means to bias said control electrode in response to the combined voltage drop across said direct-current source and bias resistor, a second source and means for controlling flow therefrom of current through said" bias resistor, and a'rectifier and a limiting resistor serially connected in shunt with said direct-current source and said bias resistor, said rectifier, being poled to prevent current flow from said direct-current source through said limiting resistor.

3. In combination with a first electrical discharge tube having a pair of principal electrodes and a control electrode, a first current source and a first resistor in series with said discharge tube, a direct-current source and a bias resistor connected in series with each other to impress a positive bias on said control electrode, connections for producing a negative bias on said control electrode by voltage drop in said bias resistor, a second voltage source and a second electrical discharge tube, said second source, bias resistor and said second electrical discharge tube being connected in series, means for impressing a control voltage onsaid second tube, a rectifier and a limiting resistor connected in series with each other to form a shunt path between the remote terminals of said bias resistor and said directcurrent source, said rectifier being poled to prevent current flovv from said direct-current source through said limiting resistor, and a capacitor in shunt relation with said first resistor proportioned to substantially discharge therethrough in a time which is long compared to the time required for said impression of control Voltage to substantially alter the bias on said control electrode.

4. In combination with a first electrical discharge tube having an anode, a cathode and a control electrode, a first current source and a first resistor in series with said discharge tube, a direct-current source and a bias resistor connected in series with each other between said cathode and control electrode, connections for producing a negative bias on said control electrode by voltage drop in said bias resistor and comprising a second voltage source and a second electrical discharge tube connected to send current through said bias resistor, means for impressing a control voltage on said second tube, a rectifier having its cathode connected to said control electrode and its anode connected through a limiting resistor to said cathode, said rectifier being poled to prevent current flow from said direct-current source through said limiting resistor, and a capacitor shunting said limiting resistor and proportioned thereto so as to have a long time constant.

5. In combination with a first electrical discharge tube having an anode, a cathode and a control electrode, a first resistor having one terminal connected to said cathode and its other terminal connected to said anode through an alternating-current source, a direct-current source having its negative terminal connected to said cathode and having its positive terminal connected to said control electrode through a bias resistor, a second electrical discharge tube having an anode, cathode and control electrode, means for causing said direct-current source to impress a negative biasing potential between the control electrode and cathode of said second tube, a second alternating-current source connected between the anode of said second tube and the control electrode of said first tube, a limiting resistor and a rectifier connected between the cathode and control electrode of the first tube, said rectifier being so poled as to prevent current flow from said direct-current source through said limiting resistor, and means for impressing a control voltage between the control electrode of said second tube and the cathode of said first tube.

6. In combination with a first electrical disin charge tube having an anode, a cathode and a control electrode, a first resistor havin one ter minal connected to said cathode and its other terminal connected to said anode through an alternating current source, a direct current source, having its negative terminal connected to said cathode and having its positive terminal connected to said control electrode through a bias resistor, a second electrical discharge tube having an anode, cathode and control electrode, means for causing said direct-current source to impress a negative biasing potential between the control electrode and cathode of said second tube, a second alternating-current source connected between the anode of said second tube and the control electrode of said first tube, a limiting resistor and a rectifier connected between the cathode and control electrode of the first tube, said rectifier being so poled as to prevent current flow from said direct-current source through said limiting resistor, and means for impressing a control voltage between the control electrode of said second tube and the cathode of said first tube, at least one of said tubes being of the gaseous discharge type.

'7. In combination with an electrical discharge tube having a pair of principal electrodes and a control electrode, a first source of alternating voltage connected to send current through said tube, a bias resistor in series with a direct current source, means to bias said control electrode in response to the combined voltage drop across said direct current source and bias resistor, a second source of alternating voltage and means including another discharge tube for controlling the flow therefrom of current through said bias resistor, and a rectifier tube and a serially connected limiting resistor in shunt with said direct current source and said bias resistor, said rectifier tube being poled to prevent current flow from said direct current source through said limiting resistor.

8. In combination with an electrical discharge tube of the gas-filled type having a pair of principal electrodes and a control electrode, a first source of alternating voltage connected to send current through said tube, a bias resistor in series with a direct current source, means to bias said control electrode in response to the combined voltage drop across said direct current source and bias resistor, a second source of alternating voltage and means for controlling flow therefrom of current through said bias resistor, and a rectifier tube and a limiting resistor serially connected in shunt with said direct current source and said bias resistor, said rectifier tube being poled to prevent current fiow from said direct current source through said limiting resistor.

ROY C. FOX. FREDERIC S. BEALE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,955,874 Demarest Apr. 24, 1934 2,036,307 Steenbeck et a1 Apr. 7, 1936 2,217,401 Geiger Oct. 8, 1940 2,299,229 I-Iall Oct. 20, 1942 2,370,287 Bivens Feb. 27, 1945 2,429,762 Koch Oct. 28, 1947 

